1. Field of the Invention
The present invention relates to a micro-electro-mechanical (MEMS) variable capacitor with reduced influence of a surface roughness, in particular for radio frequency (RF) applications, to which the following description will make explicit reference without this, however, implying any loss in generality.
2. Description of the Related Art
As is known, in the last few years MEMS devices have been developed for a wide variety of applications, in view of their size, cost and power consumption advantages. In particular, variable capacitors (the so called “varicaps”) manufactured using MEMS technology have been successfully introduced in RF applications (such as in wireless mobile communication), instead of conventional variable capacitors, such as junction diodes or MOS capacitors. For example, MEMS variable capacitors have been used in shunt antenna switches, tunable filters, and voltage-controlled oscillators.
FIG. 1 illustrates a cross section of a variable capacitor 1, of a MEMS type. In detail, the variable capacitor 1 comprises a fixed electrode 2 and a movable electrode 3, of conductive material (e.g., aluminum, gold or nickel) or a combination of a dielectric material (e.g., oxide or nitride) and a conductive material, which constitute respectively the top and bottom plates of the variable capacitor having a capacitance C. The fixed electrode 2 is arranged on, and fixed to, a dielectric layer 4 (e.g., of silicon oxide) formed on a substrate 5, for example of semiconductor material (silicon) or glass; a dielectric region 6 (e.g., of silicon oxide or nitride) locally coats the fixed electrode 2. The movable electrode 3 is a membrane, which is suspended over the fixed electrode 2, and is spaced apart from the dielectric region 6 by an interelectrode air gap 7, having a thickness dg. The movable electrode 3 is electrically connected to actuation electrodes 8 which are arranged on the dielectric layer 4 laterally to the fixed electrode 2; the actuation electrodes 8 mechanically anchor the movable electrode 3 to the substrate 5. At least part of the membrane is perforated (in a not shown manner) in order to allow releasing of the membrane by etching of a sacrificial region, during a related manufacturing process.
During operation, a dc actuation voltage Vdc is applied across the plates of the variable capacitor 1 by means of the actuation electrodes 8, resulting in an electrostatic force between the fixed electrode 2 (bottom plate) and the movable electrode 3 (top plate). This electrostatic force pulls the movable electrode 3 towards the fixed electrode 2, determining a decrease of the thickness dg of the interelectrode air gap 7 and a corresponding increase of the capacitance value; in particular, the movable electrode 3 is pulled down to a position at which an equilibrium is reached between the electrostatic force due to the applied actuation voltage Vdc and an elastic force generated in the membrane. As shown in FIGS. 2a-2b, as long as the actuation voltage remains below a critical value, generally called the pull-in voltage (denoted with Vpi), the amount of displacement of the movable electrode 3 is a result of the equilibrium between the electrostatic force and the elastic force in the membrane. In this operating region (shown in the enlarged detail of FIG. 2b), the variable capacitor 1 acts as a tuneable capacitor, and the capacitance C shows an increasing trend with the actuation voltage Vdc. When the actuation voltage Vdc exceeds the pull-in voltage Vpi, no equilibrium can be reached any more, and the movable electrode 3 collapses on the dielectric region 6 coating the fixed electrode 2, as shown in FIG. 3.
This situation is unwanted for a tuneable capacitor, but it is the normal operation of a capacitive switch, which has two operating states: the on-state, for actuation voltages below the pull-in voltage Vpi, and the off-state, for actuation voltages above the pull-in voltage Vpi. In particular, a minimum capacitance value Cmin is associated to the on-state, and a maximum capacitance value Cmax is associated to the off-state; the ratio between the maximum capacitance value Cmax and the minimum capacitance value Cmin is generally called the switching ratio (denoted with SR) of the variable capacitor 1, and is to be maximized for optimum operation of the capacitive switch (typically, the switching ratio is between 10 and 50).
An important factor that influences the maximum capacitance value Cmax (and the switching ratio) is the roughness of the facing surfaces of both the movable electrode 3 and the dielectric region 6. In particular, if surfaces were flat and without roughness, the maximum capacitance value Cmax associated to the off-state of the variable capacitor 1 would be:
      C    max    =                    ɛ        0            ·              ɛ        r              ⁢          S      d      wherein ∈0 is the absolute dielectric constant (dielectric permittivity in vacuum), ∈r is the electric permittivity of the dielectric region 6, S is the facing area of the electrodes, and d is the thickness of the dielectric region 6. In particular, this maximum capacitance value Cmax corresponds to a capacitance Cdiel due to the dielectric region 6.
However, if the above surfaces have a certain amount of roughness, as shown in the detail of FIG. 4, they do not perfectly adhere to each other and gap regions 10 filled with air are formed between the movable electrode 3 and the dielectric region 6, and an unwanted capacitance Cair due to the air that fills the gap regions 10, is generated in series to the capacitance Cdiel due to the dielectric region 6. Since the electric permittivity of air is equal to 1, the resulting value of the maximum capacitance C′max is strongly decreased by the presence of air, according to the formula:
      C    max    ′    =      (                            C          diel                ·                  C          air                                      C          diel                +                  C          air                      )  C′max being much lower than Cmax.
Accordingly, the switching ratio of the variable capacitor 1 is decreased with respect to a design value, and so are the electrical performances thereof, due to the presence of the surface roughness.